The present invention relates to an improved and more efficient method of delivering a coal-oil slurry of reduced sulfur and ash content to the combustion chamber of a boiler.
The continually escalating cost of fuel oil as an energy source and its predicted depletion makes the use of other type fossil fuels as fuel oil substitutes look attractive. The abundance of coal and its accessibility suggests immediate direct substitution of coal for fuel oil wherever possible.
Several factors, however, have retarded the immediate substitution of coal for fuel oil. One such factor is the difficulty in transporting the coal in bulk from the point of origin to the place of intended use. Another important factor to be considered in the substitution of coal for fuel oil is the effect of the coal burning by-products on the ecology. A third factor which has retarded the conversion of oil-burning facilities to coal-burning facilities is the cost of converting oil-burning equipment over to equipment for burning coal.
A compromise to direct substitution of coal for fuel oil is the so-called "coal-oil-slurry". The slurry offers a relatively cheap method for converting oil-burning equipment at least partially to coal firing, without the cost required for major boiler redesign. Furthermore, the slurry can be pumped along a pipeline, thus offering a solution to the coal transport problem.
However, one problem which remains is that of the pollution problems that result from burning most coals. In this regard, it is the sulfur in the coal which presents the most serious pollution problem. The large amount of ash in the stack gases of a coal fired plant is also detrimental to the environment. Ash also causes slagging on boiler tubes and shortens boiler life.
Sulfur in coal occurs both in the organic and inorganic forms. Organic sulfur is chemically bonded into the hydrocarbon structure of the coal and cannot generally be removed by purely physical means like magnetic separation. The inorganic sulfur occurs as iron sulfide mineral inclusions in the coal and can be removed magnetically. In addition, some of the non-sulfur bearing mineral components, which would ultimately form ash, are also magnetic to a degree and can be removed magnetically.
The relative proportions of organic and inorganic sulfur in coal vary with the source of the coal. In many coals from the eastern half of the United States, the proportions are approximately equal; removal of the inorganic component reduces the total sulfur content by half.
The iron sulfides which comprise the inorganic sulfur in coal occur as both pyrite, FeS.sub.2, and pyrrhotite whose composition is approximately Fe.sub.O.9 S. Most of the sulfide is pyrite, which in its pure state is hardly magnetic at all. Interlocked with the pyrite, however, are small amounts of pyrrhotite which are rather strongly magnetic and account for the fact that the pyrite can be removed by magnetic separation.
One well-known approach to improve the removal of pyrite in coal is to convert it to the more magnetic pyrrhotite before magnetic separation. Thus, in the prior art procedure, the coal was pulverized and heated to an appropriate temperature for conversion of pyrite to pyrrhotite. Some of the sulfur is released into the atmosphere during the process and provision must be made to collect this to meet environmental requirements. The remainder in the form of pyrrhotite may be extracted magnetically. The heating process should be selective, i.e., the pyrite and not the coal should be heated. In the prior art microwave heating has been employed for this purpose. See U.S. Pat. No. 3,463,310 to Ergun et al. entitled Separation Method. This heating step is energy intensive and the heat is not recovered.
Another method of pyrite removal in the prior art is to pulverize the coal, omit any special pretreatment, slurry the pulverized coal in water and pass the slurry through a magnetic separator. The pyrite particles are retained and the slurried coal passes through. The disadvantage of this technique is that the coal must then be dried or else burned wet; thus each alternative entails a loss of energy.
Although the pulverized coal may be slurried in oil instead of water and then passed through a magnetic separator without pretreatment, as in the case of water slurries, some serious difficulties arise. The heavy fuel oils used in power plant practice are very viscous, even when heated to 130.degree. F., a temperature commonly used for injection into the boiler. Viscosities of the order of hundreds of centipoises are common compared to 1 centipose, the viscosity of water. The effectiveness of magnetic separation decreases directly with the viscosity of the carrier fluid, so the degree of pyrite removal would be poor compared to that with the water slurries.
Another disadvantage of the method of the prior art is that the pyrrhotite component of the pyrite is not always in the optimum magnetic state. Although all pyrrhotite is more strongly magnetic than pyrite, there is an optimum form which is very strongly magnetic. This is the monoclinic form which occurs in a narrow range of stoichiometries in the neighborhood of the composition Fe.sub.7 S.sub.8. The less strongly magnetic form is hexagonal pyrrhotite and is more common. In the vicinity of 220.degree. C. hexagonal pyrrhotite converts to the monoclinic form. By raising the temperature to 220.degree. C. and then performing a magnetic separation at this temperature, a more complete removal of the sulfides in coal can be achieved. At this temperature the viscosity of the oil is lower by about a factor of 50, thereby further assisting in the magnetic separation process. A certain fraction of the ash-forming minerals in the coal, other than the iron sulfides, are paramagnetic and are also removed in the magnetic separation step. A reduction in the ash component of 20 to 40% is often achieved.